Disentangling the Effects of Climate and Landscape Change on Bird Population Trends in the Western U.S. and Canada

Preparing for global warming means understanding both the likely future changes to patterns of weather and also what those changes mean for wildlife. Recent work funded in part by the Northwest Climate Science Center shows that for many birds across the West, changes in patterns of precipitation matter more than changes in temperature. These results have important consequences for managing not only birds, but entire ecosystems, because of the roles birds play as pollinators, seed dispersers, and insect predators.

The blurred wings and brilliant feathers of the Rufous hummingbird are a popular, but increasingly rare, site at backyard feeders from California to Alaska. In recent years populations of Rufous hummingbirds have declined by an average of 3% annually across their range. Dr. Matthew Betts of Oregon State University’s College of Forestry and his colleagues found that these declines link to reductions in December precipitation- a pattern that also holds for many other bird species across western North America.

“When we think of climate change, we automatically think warmer temperatures,” said Betts, in a recent press release. “But our analysis found that for many species, it is precipitation that most affects the long-term survival of many bird species.

“It makes sense when you think about it,” Betts added. “Changes in precipitation can affect plant growth, soil moisture, water storage and insect abundance and distributions.”

Betts and his colleagues, Javier Guttierrez Illan, Chris Thomas, Julia Jones, Weng-Keen Wong, Susan Shirley from Oregon State University and University of York, examined long-term data on bird distributions and abundance across five states in the western United States, and British Columbia, testing statistical models to predict population changes for 132 species over 32 years. They analyzed the impacts of temperature and precipitation on bird distributions at the beginning of the study period (the 1970s) and then tested how well the predictions forecast actual population trends over the subsequent 30 years.

The scientists narrowed their focus to several variables, including possible changes during the wettest month in each region, the breeding season of different species, and the driest month by area. Their model found that variables relating to precipitation were most useful in predicting bird population trends. December precipitation turned out to be one particularly influential variable.

“Someone might ask why December, since half of the bird species usually present in the Pacific Northwest, for instance, might not even be here since they’re migratory,” said Betts. “But much of the critical precipitation is snow that falls in the winter and has a carryover effect for months later – and the runoff is what affects stream flows, plant growth and insect abundance well down the road.”

The ability of the model to predict population changes varied greatly from species to species- from 80% accurate to no better than random.

“We cannot say for certain that a change in December precipitation caused declines in rufous hummingbirds,” said Javier Gutiérrez Illán, a former postdoctoral researcher at Oregon State and lead author on the study. “Our model shows, however, a strong association between the birds’ decline and precipitation changes and the fact that this variable pointed to actual past changes in populations gives it validity.”

The recent study is one of only a few to consider the role of precipitation patterns in driving changes to wildlife distribution. Its results indicate that other models focused on future habitat suitability could benefit from a similar approach.

Climate change has been implicated in the range shifts and population declines of many species, but the confounding of climate change with other variables, particularly landscape change, hampers inference about causation. Climate envelope models have been used to predict population trends and future distributions, but the reliability of such predictions remains relatively unknown; without tests of model accuracy, policy development will be based on highly uncertain ground. Our team assembled recent developments in change detection mapping and species modeling. Specifically, our objectives were to: (1) use 32-year data on bird distributions to test the reliability of climate envelope models, (2) test whether changes in climate are linked to bird population declines over the past 32 years and, (3) assess the relative importance of climate versus landscape change in explaining changes in species distributions.

Some bird species are more dependant on temperature, others on land use changes: This project used historical bird distribution and imagery to test whether climate, habitat change, or a combination of the two altered bird species abundance. The results can help managers prioritize conservation efforts on birds with the greatest potential declines.